Dashboards - fix optionality of SearchEngines

Summary: Ref T7234. I didn't know about this spot in D11750.

Test Plan: ..the next diff really makes this work for the T7234 scenario.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7234

Differential Revision: https://secure.phabricator.com/D11751

src/applications/dashboard/customfield/PhabricatorDashboardPanelSearchApplicationCustomField.php

@@ -16,6 +16,7 @@ final class PhabricatorDashboardPanelSearchApplicationCustomField
     $engines = id(new PhutilSymbolLoader())
       ->setAncestorClass('PhabricatorApplicationSearchEngine')
       ->loadObjects();
+    $engines = mfilter($engines, 'canUseInPanelContext');
     $all_apps = id(new PhabricatorApplicationQuery())
       ->setViewer($this->getViewer())
       ->withUnlisted(false)